Referring to an ass, usually a female's of great quality. Said donk is large, round, and firm not flabby.
Origination: It is the short-hand for donkey, which is of course a synonym of ass through animal nomenclature.
Other: The term became exceedingly popular after Soulja Boy's song by the same name was released.
A girl with a fine rump walks by...
Boy 1 (pointing) : She got a donk!
Boy 2: Damn dude, she does have a donk.